In recent years, N-heterocyclic carbenes (NHCs) have attracted considerable interest since the isolation of stable imidazol-2-ylidene by Arduengo in 1991 (Arduengo III, A. J.; Harlow, R. L.; Kline, M. K. J. Am. Chem. Soc. 1991, 113, 361; Arduengo III, A. J. Acc. Chem. Res. 1999, 32, 913.). The similarity of electron-rich organophosphanes PR3 and NHCs, and NHCs' excellent σ-donating properties make them ligands of choice for transition metals. This has led to the preparation of organometallic catalysts which are useful in organic synthesis. It was found that a number of reactions, such as C—C coupling and olefin metatheses were more effectively catalyzed by NHC-metal complexes than by conventional catalysts. However, these homogeneous catalyst systems are difficult to separate and recycle, and pose a contamination issue for the products.
Compared to homogeneous catalysts, heterogeneous catalysts can be easily separated from the products and reused, and are attractive for green chemistry. Recently, several types of supported NHC-transition metal complexes have been designed so as to combine the advantages of both homogeneous and heterogeneous catalysts. NHC-metal complexes supported on mesoporous materials and particles/polymer hybrid materials have also been developed for various reactions. However, the existing polymer-supported or mesoporous materials-supported catalysts are limited by low activity, multi-step synthesis and low catalyst loading. The polymer-supported catalysts suffer from polymer swelling and the mesoporous silica-supported catalysts are sensitive to basic or acidic reaction conditions.
Micro- and nanometer-sized spherical particles have long been recognized for their many applications, including catalysis, optics, biosensing, drug delivery and data storage. Different methods have been developed for preparation of three major classes of materials particles: organic polymer particles, inorganic materials particles and very recently coordination polymer particles. Inorganic particle materials have had great impact on catalysis applications, but organic particles or coordination polymer particles directly used as catalysts in catalysis applications are still an undeveloped area. Only limited monomers and reactions can be used for preparation of organic and coordination polymer particles. Another class of materials are organometallic compounds, which are characterised by metal-carbon bonds and are well known as important catalyst materials in catalysis. Although organometallic compounds are widely used as precursors for preparation of inorganic particles, they have so far not yet been prepared as micro- or nanometer-sized colloidal particles. On the other hand, to tailor the chemical properties of particles for catalysis applications is still a great challenge.
Many efforts have been made to develop heterogeneous NHC organometallic catalysts through immobilization methods. However, the development of solid poly-NHCs polymer is hitherto, surprisingly, an undeveloped area.